1. Field of the Invention
The present invention relates to a control apparatus for a vibration wave motor which causes relative movement between a vibration member and a member in press contact with the vibration member by generating a travelling vibration wave in the vibration member.
2. Related Background Art
A vibration wave apparatus (vibration wave motor) using a travelling vibration wave utilizes the following principle. A vibration member (stator) is constituted by fixing two groups of electro-mechanical energy conversion elements, i.e., piezoelectric elements (either a single-plate element or a plurality of elements can be divided into groups) arranged in the circumferential direction on one surface of a ring-like, elliptical, or oblong elastic member having a total length equal to an integer multiple of a given length .lambda.. These piezoelectric elements are arranged at a pitch of .lambda./2 within each group to alternately have opposite expansion-shrinkage polarities. The two groups have a shift relative to each other equal to an odd multiple of .lambda./4.
Electrodes films are respectively formed on the two groups of piezoelectric elements. When an AC voltage is applied to only one of either group (to be referred to as phase A hereinafter), the whole elastic member generates a standing wave (wavelength .lambda.) of flexural vibration having antinodes at the central point of each piezoelectric element of the group and points from the central point at a pitch of .lambda./2 or having nodes at the central points between these antinodes.
When an AC voltage is applied to only the other group (to be referred to as phase B hereinafter), a standing wave is similarly generated, but its antinodes and nodes are shifted from those of the standing wave generated by phase A by .lambda./4. If AC voltages having the same frequency and a time phase difference of 90.degree. are simultaneously applied to both phases A and B, the elastic member generates a travelling wave (wavelength .lambda.) of flexural vibration that vibrates in the circumferential direction as a result of the synthesis of standing waves by the two phases. At this time, each point of the elastic member having a thickness elliptically moves.
When, e.g., a ring-like moving member, such as a rotor, is in press contact with one surface of the elastic member, the moving member rotates upon reception of a circumferential frictional force from the elastic member. If a plurality of radial grooves are formed in the circumferential direction in the surface of the vibration member opposite the piezoelectric element fixing surface in order to increase the circumferential components of elliptical movement, the neutral plane of vibration moves to the piezoelectric element fixing surface to increase both the rotational number and the motor efficiency with the same amplitude. These grooves also function to remove any wear powder. From this principle, the vibration wave apparatus has the following advantages:
1) The vibration wave apparatus has a holding torque during no current supply, and does not cause hunting during positioning. PA1 2) The vibration wave apparatus rotates quickly and stops rotating quickly (small mechanical time constant) due to a small inertia and a large driving torque. PA1 3) The vibration wave apparatus is free from any cogging because all the points on the circumference generate the same driving force. PA1 1) The sliding material must have a sufficiently large frictional coefficient (in order to increase the motor torque). PA1 2) The sliding material must have a sufficiently high wear resistance (in order to maintain the motor performance over time). PA1 3) The sliding material must have a sufficiently high thermal conductivity (in order to improve heat dissipation and prevent temperature changes in motor characteristics). PA1 4) The moving member must be lightweight (in order to decrease the inertia). PA1 1) the fixation phenomenon occurs with a combination of materials producing a wear powder about 1 pm or less in minimum size upon driving, and PA1 2) the fixation phenomenon occurs upon driving for a given time. PA1 1) the fixation phenomenon depends on the relative humidity rather than the absolute humidity (temperature dependency is small at 0.degree. C. or higher), and PA1 2) the fixation phenomenon occurs upon abrupt changes in temperature. PA1 (1) Contamination of the surface in processing and pre-driving environments is removed by sliding. PA1 (2) The contact stress is unified (surface roughness is unified and a nonuniform contact state is improved). PA1 (3) A chemical equilibrium is attained (chemical changes by the temperature of the sliding surface, work-hardening by the stress, and chemical or physical adsorption of the surface determined by the above surface states and the atmosphere reach a steady state).
The vibration wave apparatus therefore is theoretically suitable for high-precision rotation and high-precision positioning. However, since the vibration wave apparatus uses a frictional force, variations in motor performance are inevitable owing to wear of the frictional surface over time or changes in the frictional force with changes in humidity or temperature.
Based on these advantages and disadvantages, a sliding material must satisfy the following requirements:
To satisfy conditions 1) and 3), the sliding material is desirably a combination of inorganic materials. However, inorganic materials have poor wear resistance when used as a material for a vibration wave apparatus, and applications of the vibration wave apparatus are limited.
In general, a combination of sliding materials for the vibration wave apparatus is proposed as a combination of a composite resin, which exhibits relatively low wear, and an inorganic material. Examples of the composite resin are resins prepared by providing, in a heat-resistant resin used as a base material, a fiber such as carbon fiber, glass fiber, and aromatic polyamide fiber, a whisker such as silicon nitride, silicon carbide, or potassium titanate, a filler having a reinforcing action or an action of increasing the thermal conductivity, and a filler having a lubricating action such as PTFE resin, molybdenum disulfide, or graphite.
Examples of the inorganic material are a raw or surface-hardened metal of stainless steel or phosphor bronze, a tungsten, nickel, cobalt, or chrome plating layer, a material prepared by filling or precipitating silicon carbide for reinforcement or a fluoroplastic for stabilizing the coefficient of friction in the plating layer, various oxide and carbide ceramics and metals, and a composite sprayed material. Some of these materials are practically available.
Some combinations of composite resins and inorganic materials exhibit high wear resistance. However, when such composite resin and inorganic material are used for steady sliding surfaces to a certain degree upon driving by a driving apparatus, they are fixed to each other (to be referred to as a fixation phenomenon hereinafter) in a humid environment.
This fixation phenomenon is analyzed in various references. For example, the fixation phenomenon is chemically explained by a theory of resin hydrolysis in Japanese Patent Application Laid-Open No. 6-261556 and a theory of metal ion presence in Japanese Patent Application Laid-Open No. 5-137358.
However, in experiments, a composite resin and inorganic material excellent in motor performance and wear resistance (e.g., wear of about several .mu.m upon driving of 100 hrs) are always fixed to each other.
This fixation phenomenon is experimentally confirmed to occur even with a slight difference in fixing force regardless of wettability of the surface of the base material of the composite resin, a thermosetting resin, a thermoplastic resin, crystallinity, a material (polytetrafluoroethylene, polyether ether ketone, polyether sulfone, polyamide, or polyimide), a filler (carbon fiber, carbon bead, potassium titanate whisker, or molybdenum disulfide), and the inorganic material (type of metal or ceramic (carbide or oxide)) of the combination. Note that even analysis upon experiments could not determine hydrolysis of the resin.
As a result of various examinations of conditions under which the fixation phenomenon occurred, conditions on the vibration wave apparatus side are
Environmental conditions are
In the vibration wave apparatus, therefore, the fixation phenomenon is supposed to be caused by sliding surfaces very close to each other without the mediacy of any large wear powder, and by microscopic condensation as an external factor.
This phenomenon is inferred as follows. When the frictional portion is driven to a given degree, the sliding surfaces of the vibration member and moving member "drape". This drape is exemplified as changes in contact surface:
(2) Unifying the contact stress simultaneously unifies the surface roughness to a certain degree. Roughly speaking, the surface roughness in a steady sliding direction represents that wear upon sliding occurs in the state of wear powder particles having a size corresponding to surface roughness level. The steady surface roughness and wear amount correlate with each other depending on the strength of the material.
That is, the surface roughness of a durable sliding member is small, and the contact area increases in correspondence with a decrease in distance between two sliding surfaces. The contact area of a material such as a resin which readily creeps increases.
For this reason, when the vibration wave apparatus is placed in a humid environment or experiences an abrupt temperature drop even in the presence of a slight absolute humidity, condensation occurs on portions of the sliding surfaces close to each other, and an intermolecular force acts to microscopically fix the sliding surfaces to each other. The fixed sliding surfaces cannot be separated even by drying due to the following reason. Water molecules entering the gap between the two adjacent surfaces are held by adjacent space enclosing surfaces and in some cases by a Van der Waals force between wear powder particles smaller in size than the distance between the adjacent surfaces. Even if the vapor pressure in a slightly open space decreases, the water molecules are not separated. When resin is employed, the slightly open space is further narrowed by creeping of the material, and the fixation becomes stronger. The fixation makes it difficult to activate the vibration wave apparatus and he like.